1. Field of the Invention
This invention relates to a voltage transforming circuit for generating different voltages from an input voltage, and more particularly to a circuit of this kind which is incorporated in an integrated circuit.
2. Description of the Related Art
In the field of compact business machines such as an electronic calculator and a personal digital assistant, liquid crystal display units are employed to save power or to make the machine compact. A liquid crystal display unit uses AC signals in order to lengthen its life. It is necessary to apply different voltages to create AC signals. The different voltages are generally created from a single source voltage by means of a voltage transforming circuit.
FIG. 1 is a block diagram, showing a conventional integrated voltage-transforming circuit used for such a purpose. This circuit transforms the output voltage VDD of a battery 11 into four different voltages Vreg (VLC1), VLC2, VLC3, VLC4. The high-potential-side voltage VDD and the reference-potential-side voltage GND of the battery 11 are applied via external terminals 12 and 13 to the integrated circuit including the voltage transforming circuit. In the integrated circuit, a predetermined voltage Vreg lower than the voltage VDD is created from the voltage VDD by means of a constant-voltage regulator 14. The voltage VDD is input to a level shifter 15. The level shifter 15 further receives a voltage VLC4, which is the highest one of voltages generated by a step-up circuit explained later, and a clock signal CK1 supplied via the external terminal 16 from the outside of the integrated circuit. The level shifter 15 shifts the level of a high-level portion of the clock signal CK1 from the voltage VDD to the voltage VLC4, thereby outputting a clock signal CK2 with an amplitude between the voltages VLC4 and GND. The clock signal CK2 obtained by level shifting of the level shifter 15 is supplied to a step-up circuit 17, together with the voltage Vreg. The step-up circuit 17 steps up the voltage Vreg to create a stepped-up voltage VLC2 two times the voltage Vreg, a stepped-up voltage VLC3 three times the voltage Vreg, and a stepped-up voltage VLC4 four times the voltage Vreg. The step-up circuit 17 is a well-known charge pump circuit of a capacitor-coupling type using capacitors, and generates a desired stepped-up voltage by connecting capacitors 41-44 to external terminals 18-21. The external terminals 18-21 are connected to an internal circuit 40, such as a circuit for generating a liquid crystal-driving signal, to apply thereto a driving voltage. The clock signal CK2 is used as a synchronization signal for controlling the step-up operation of the step-up circuit 17.
Further, it is necessary to provide capacitors outside the integrated circuit so as to cause the step-up circuit 17 to perform the step-up operation. For this purpose, the external terminals 18-21 are provided. However, it is known that a surge voltage such as static electricity may be applied to the external terminals of the integrated circuit. Since the external terminals 18-21 are directly connected to internal elements of the integrated circuit, such as transistors employed in the step-up circuit 17, it is possible that the step-up circuit 17 and the internal circuit will be broken when such a surge voltage has been applied to the external terminals 18-25. To prevent this, protect elements for protecting the internal circuit from the surge voltage are connected to the external terminals 18-21. In the case of a MOS-type integrated circuit which employs MOS transistors used as active elements, P-channel MOS transistors 22-24 and N-channel MOS transistors 25-28 are used as protect elements, as shown in FIG. 1. Each of these MOS transistors has its gate, source and back gate connected to each other, and an element equivalent to a diode is formed between the connection node and the drain.
The operation of the conventional circuit constructed as above, assumed when the relationship of VLC4&gt;VDD&gt;Vreg is established and the step-up circuit 17 is operating in a stable manner, will be explained. Suppose that a compact business machine such as an electric calculator or a personal digital assistant driven by a battery of 3 V is used. Further, suppose that the voltage Vreg is 1.5 V, VLC4 6 V (1.5.times.4), the voltage VDD 3 V, the minimum operation-guarantee voltage of the level shifter 15 (hereinafter referred to as "VDDmin") 1.2 V, and the forward voltage of each diode (hereinafter referred to as "VF") 0.5 V. In a state assumed immediately after the step-up circuit 17 starts the step-up operation, the voltage Vreg is higher than the voltage VLC4, and a forward current flows from the Vreg terminal to the VLC4 terminal via the P-channel MOS transistor 22 as a protect element. Thus, an initial voltage is applied to the VLC4 terminal.
The above-described protect element, however, raises the following problems:
FIG. 2 shows the input waveform (CK1) of the level shifter 15, and the output waveform (CK2) of the same obtained after the step-up operation of the step-up circuit 17 is stabilized. Further, FIG. 3 shows the input waveform (CK1) of the level shifter 15 obtained in an initial state immediately after the step-up operation of the step-up circuit 17 starts, as well as the voltages VDD, VCL4, Vreg and VDDmin. As is shown in FIG. 3, in the initial state, the voltage VLC4 is lower than the voltage Vreg, and therefore a forward current flows from the Vreg terminal to the VLC4 terminal. At this time, the VLC4 terminal has a potential of 1 V lower than the voltage Vreg (1.5 V) by the voltage VF (0.5 V) of the diode. Therefore, where the voltage VLC4 (=Vreg-VF) is lower than the voltage VDDmin of the level shifter 15, for example, immediately after the integrated circuit is turned on, the level shifter 15 does not operate, and the step-up circuit 17 which is controlled by the clock signal CK2 output from the level shifter 15 does not operate.
To avoid such inconvenience, the output voltage Vreg of the constant-voltage regulator 14 must be set higher than 1.5 V. However, in order to cause the integrated circuit to be operable under a wide range of input voltages, for example, when the voltage VDD ranges from 1.8 V to 3.3 V, the output voltage Vreg of the constant-voltage regulator 14 must be set to approx. 1.5 V. If the voltage Vreg is set higher than 1.5 V, the lower limit of the input voltage is limited to a value higher than in the case of the Vreg being 1.5 V, which degrades the versatility of the integrated circuit. Moreover, there is a case where the voltage Vreg is calculated from the specification related to the voltages VLC2, VLC3 and VLC4.
As described above, in the conventional voltage transforming circuit in which a plurality of voltages are obtained by stepping up the input voltage, an initial voltage is supplied via the protect elements for protecting the internal circuit from a surge voltage applied to the external terminals. With such an initial voltage, it is possible that the voltage transforming circuit is not normally operated at the time of turning the integrated circuit on. In this case, desired output voltages cannot be obtained.